Question

A 6.1 g copper coin is given a charge of −4.0 ✕ 10−9 C.
(a)
How many excess electrons are on the coin?
[2.5E10]
(b) By what percent do the excess electrons change the mass of the coin?

I already know a is correct, I'm struggling with b. I keep getting 3.733E-18 by dividing the excess electrons by the mass of the coin and don't know how to translate it into percentage.

Answer 1

assuming that part A is correct at 2.5 * 10^10 electrons
mass of electron = 9.11*10^-31 kg = 9.11*10^-28 grams
divide by 6.1 and multiply by
2.5*10^10 * (9.11/6.1) *10^-26 = about 3.7*10^-16
You need to multiply the fraction by 100 to get percent

Answer 2

the whole is 100 %
1/2 or 0.5 of the whole would be 50%
0.5 * 100 = 50%